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Scott Holmes, Kendal Marriott, Alisha Mackenzie, Maggie Sin, Matthew Heath; Distinct visual metrics support the late stages of aperture shaping for 2D and 3D target objects. Journal of Vision 2012;12(9):1096. doi: 10.1167/12.9.1096.
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An issue of continued debate in the visuomotor control literature is whether a 2D target serves as a representative proxy for a 3D target in understanding the nature of the visual information supporting grasping control. For example, some studies have shown that absolute (i.e., Euclidean) visual metrics support both 2D and 3D grasping whereas other have not (c.f. Westwood et al. 2002 vs. Castiello 1998). In an effort to reconcile this issue, we applied the psychophysical principles of Weber’s law and the computation of just-noticeable-difference (JND) scores to examine the aperture shaping profiles for 2D and 3D target grasping. In particular Weber’s law states that changes in a stimulus that will be ‘just noticeable’ are a constant ratio of the original stimulus, thus, adherence and violation of JNDs to object size reflect the use of relative and absolute visual metrics, respectively. Participants grasped differently sized 2D and 3D objects (20, 30, 40, and 50 mm of width) and we computed the within-participant standard deviations of grip aperture (i.e., the JNDs) at decile increments of normalized grasping time. In terms of the early stages of aperture shaping, both 2D and 3D targets produced a linear scaling of JNDs to object size (i.e., Weber’s law). Later in the response, 2D target objects showed a continued JND/object size scaling whereas 3D objects did not. Thus, results suggest that grasping a 2D target is mediated by a unitary and relative visual percept of object size whereas the early and late stages of aperture shaping for a 3D target are respectively subserved via relative and absolute visual information. We believe that such findings add importantly to the visuomotor control literature insomuch as they demonstrate that distinct visual metrics support the later stages of grasping 2D and 3D targets.
Meeting abstract presented at VSS 2012
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